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Tests had proved that an Imperial Airways' Empire flying-boat could achieve a transatlantic crossing only if its entire payload consisted of fuel. Since it is well known that an aircraft can be flown at a much greater weight than that at which it can take off from the ground, Robert Mayo proposed that a small heavily loaded mailplane be carried to operational altitude above a larger 'mother plane' and then released to complete its long-range task. The proposal was accepted by the Air Ministry and Imperial Airways, which jointly contracted Shorts to design and build such a composite unit.

The Short S.21 Maia, the lower component, was a slightly enlarged and modified version of the Empire boat; the Short S.20 Mercury, the upper long-range unit, was a new high-wing twin-float seaplane with four 254kW Napier Rapier H engines giving a cruising range of 6116km with 454kg of mail.

This eight-engined part-time biplane composite was first tested on 4 January 1938. During take-off and before separation Mercury's flying controls were automatically locked in the neutral position, Maia's pilot having full command; the parasite's engines were started from inside the mother ship and combined with those of Maia to get the two components airborne.

The first airborne separation took place on 6 February 1938, over Rochester, Kent, and after a number of experimental flights Mercury was air-launched over Foynes Harbour, County Limerick, Ireland crewed by Captain Donald Bennett, on 21 July 1938. Mercury carried 5455 litres (1200 Imperial gallons) of fuel in its wings and 508 kg (1120 lb) of newspapers, mail and newsreel footage in her twin floats. Bennett flew on to Montreal nonstop, cover-ing the 4715 km (2930 miles) from Ireland in 13 hours 29 minutes, then set off again for New York, where for the first time ever English newspapers were on sale at the news-stands on the day after publication.

From 6 October 1938 Mercury and Bennett made news again with a nonstop flight of 9728 km (6045 miles) from Dundee, Scotland to Orange River, South Africa in 42 hours 5 minutes. The Composite subsequently operated a scheduled nonstop mail service between Southampton and Alexandria, Egypt which continued until the outbreak of World War II.

Mercury was eventually broken up at Rochester and Maia destroyed by enemy action during May 1941.